The goal of this project is to determine the role of actomyosin structural dynamics in the molecular mechanism of force generation in muscle. Emphasis is placed on the use of high-resolution and time- resolved (TR) spectroscopy to test and revise detailed mechanistic models for the functional interaction of myosin and actin. Several hypotheses inform all aims: (a) Crystal structures and electron micrographs of myosin and actin provide high-resolution models for the structural dynamics of actomyosin, which must be tested and revised by site-directed spectroscopy in functional protein complexes, using computational simulations to connect models with experiment. (b) The weak-to-strong (W-S) transition is fundamental and occurs in both actin and myosin. (c) To understand the physical basis of these transitions, the weak and strong states of actin and myosin must be characterized by dynamics and disorder as well as structure. There are five aims: (1) Development of improved methods for site-directed spectroscopic analysis of myosin and actin structural dynamics. New spectrometers, recently acquired or developed, have raised this project to a new technological level: Pulsed electron paramagnetic resonance (EPR), high-frequency EPR, and high- throughput pulsed fluorescence, performed under both steady-state and transient biochemical conditions, opens new opportunities to measure angles and distances, and to resolve structural states and transitions between them. Spectroscopy will be coordinated with collaborative studies of computational simulations, x- ray crystallography and electron microscopy (EM). (2) Structural dynamics of the myosin catalytic domain will be probed to test and revise mechanistic models. Focusing on three distinct regions (force-generating, actin-binding cleft, nucleotide pocket) and performing experiments under transient conditions, coordination of motor parts will be analyzed in space and time. Functional mutations will be used to distinguish weak and strong states. (3) Structural dynamics of the light chain (LC) domain will be probed to characterize the previously unknown structure of the N-terminal phosphorylation domain (PD) of the myosin regulatory light chain (RLC), and to determine its internal and global dynamics as affected by phosphorylation, ATP, and actin. (4) Actin structural dynamics, both global and internal, will be perturbed by weak and strong interaction with myosin, including myosin isoforms and functional mutants, to correlate structural dynamics with function. (5) The actomyosin interface will be mapped by probes on both proteins, and EM will be used to integrate the data into structural models for the weak and strong states. This work is of fundamental importance for understanding muscle function, and the technology and concepts generated here are already being used elsewhere, by this group and others, to provide insight into muscle malfunction and therapy at the molecular level. More generally, the lessons learned in this project are applicable to a wide range of problems in the biophysics of cellular movement.